Why, lightweight, they breathe the best of ANY mask (except the e-flex, I believe those would be dead even.). And customizable. I play with boat bottoms on cold days, regular on warm, put my flex lens and top into a proteus mask, wicked cool no matter how you look at it.

Soft ears, hard ears, big ears, small ears. Single strap, double strap them, cheap, easy to find parts or lenses of any variety (mirrored, fire, smoke, HD yellow, etc all in thermal). The thermal don't fog on you, the foam lasts longer then any other mask.

They protect your eyes and the bottoms give you more bounces,

The only mask that is cooler are the angel eyes, and those will never come out now.

Why, lightweight, they breathe the best of ANY mask (except the e-flex, I believe those would be dead even.). And customizable. I play with boat bottoms on cold days, regular on warm, put my flex lens and top into a proteus mask, wicked cool no matter how you look at it.

Soft ears, hard ears, big ears, small ears. Single strap, double strap them, cheap, easy to find parts or lenses of any variety (mirrored, fire, smoke, HD yellow, etc all in thermal). The thermal don't fog on you, the foam lasts longer then any other mask.

They protect your eyes and the bottoms give you more bounces,

The only mask that is cooler are the angel eyes, and those will never come out now.

Pretty much this. Actually I think the Proflexes breathe better than the E-Flexes. That is why I run Proflex bottoms on my E-Flex. Best of both worlds. E-Flex offers much better protection though. It only takes getting shot directly in the ear once with the Proflexes. Ouch.

Two questions: When people say "flexes" does it usually mean Proflex or the E-Flex?

And is that the setup most people run for a mask? -- Empire E-flex Mask but Proflex bottoms?

When players say "flexes" they are referring to the original JT proflex. Most will say E-Flexes if that is what they are referring to.

For the second question, most just wear the E-Flexes the way they come because the bottoms are more sturdy and are lower profile. I just run the proflex bottoms because they breathe better. They are a little wider so do not look as clean and they offer less protection but I will sacrifice both for the breathability.

Here is yet another one of those "go try out ... and see which one you like" questions.
I did this and found that I was most comfortable in the flexes and they covered my broad square jaw the best. I also love the way they breathe and I've only gotten them to fog once and that was an extreme case on a day where everyone was fogging up. They only fogged for a minute after sprinting the field to my favorite "Alamo" attack bunker and then sitting there and waiting for the shots to subside.
My last pair were e-vents and my next will more than likely be blue e-flexes with mirror lenses because I also like the way they fit and breathe and the ear covers are sweet. I just couldn't justify the price difference when I got my proflexes.
My. 02

Proflex - Cheapest - Lightest - Most comfortable - Most breathable - Most bounces - most customizable - they are also as rugged as any other mask out there - less likely to fog due to high breath ability - You can also yell really loud and not feel like your yelling in your own ears, which is how I feel in most masks. E-Flex has easier to change lense, almost double the cost, and pinches my nose, and much less customizable if you care. I wouldn't even consider the rest of the masks listed but thats just me.